A. Karastergiou
A5060268947- Pulsars and Gravitational Waves Research
- Radio Astronomy Observations and Technology
- Astrophysics and Cosmic Phenomena
- Geophysics and Gravity Measurements
- Gamma-ray bursts and supernovae
- Advanced Frequency and Time Standards
- Astrophysical Phenomena and Observations
- GNSS positioning and interference
- Geophysics and Sensor Technology
- High-pressure geophysics and materials
- Superconducting and THz Device Technology
- Astronomical Observations and Instrumentation
- Galaxies: Formation, Evolution, Phenomena
- Antenna Design and Optimization
- Astronomy and Astrophysical Research
- Radio Wave Propagation Studies
- Particle Accelerators and Free-Electron Lasers
- Particle Detector Development and Performance
- Solar and Space Plasma Dynamics
- Ionosphere and magnetosphere dynamics
- Stellar, planetary, and galactic studies
- Superconducting Materials and Applications
- Inertial Sensor and Navigation
- Cold Atom Physics and Bose-Einstein Condensates
- Scientific Research and Discoveries
University of Oxford
2016-2025
Rhodes University
2015-2025
University of the Western Cape
2015-2022
SKA Telescope, South Africa
2022
University of Toronto
2016
Institute of Astronomy and Astrophysics, Academia Sinica
2016
National Institute for Theoretical Physics
2016
Max Planck Institute for Radio Astronomy
2000-2015
Max Planck Society
2001-2015
University of Amsterdam
2010-2011
LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in north of Netherlands and across europe. Utilizing novel phased-array design, LOFAR covers largely unexplored low-frequency range from 10-240 MHz provides number unique observing capabilities. Spreading out core located near village Exloo northeast Netherlands, total 40 stations are nearing completion. A further five have been deployed throughout Germany, one station has built each France, Sweden, UK....
The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is a new 400-800MHz radio interferometer under development for deployment in South Africa. HIRAX will comprise 1024 six meter parabolic dishes on compact grid map most of the southern sky over course four years. has two primary science goals: to constrain Dark Energy measure structure at high redshift, study transients pulsars. observe unresolved sources neutral hydrogen via their redshifted 21-cm emission line (`hydrogen...
The discovery that at least some Fast Radio Bursts (FRBs) repeat has ruled out cataclysmic events as the progenitors of these particular bursts. FRB~121102 is most well-studied repeating FRB but despite extensive monitoring source, no underlying pattern in repetition previously been identified. Here, we present results from a radio campaign using 76-m Lovell telescope. Using pulses detected data along with literature, report detection periodic behaviour source over span five years data. We...
Abstract We describe system verification tests and early science results from the pulsar processor (PTUSE) developed for newly commissioned 64-dish SARAO MeerKAT radio telescope in South Africa. is a high-gain ( ${\sim}2.8\,\mbox{K Jy}^{-1}$ ) low-system temperature ${\sim}18\,\mbox{K at }20\,\mbox{cm}$ array that currently operates 580–1 670 MHz can produce tied-array beams suitable observations. This paper presents MeerTime Large Survey Project commissioning with PTUSE. Highlights include...
Low frequency radio waves, while challenging to observe, are a rich source of information about pulsars. The LOw Frequency ARray (LOFAR) is new interferometer operating in the lowest 4 octaves ionospheric "radio window": 10-240MHz, that will greatly facilitate observing pulsars at low frequencies. Through huge collecting area, long baselines, and flexible digital hardware, it expected LOFAR revolutionize astronomy frequencies visible from Earth. next-generation telescope pathfinder Square...
Faraday rotation measurements using the current and next generation of low-frequency radio telescopes will provide a powerful probe astronomical magnetic fields. However, achieving full potential these requires accurate removal time-variable ionospheric contribution. We present ionFR, code that calculates amount for specific epoch, geographic location, line-of-sight. ionFR uses number publicly available, GPS-derived total electron content maps most recent release International Geomagnetic...
Pulsars emit low-frequency radio waves through to high-energy gamma-rays that are generated anywhere from the surface out edges of magnetosphere. Detecting correlated mode changes in multi-wavelength emission is therefore key understanding physical relationship between these sites. Through simultaneous observations, we have detected synchronous switching and X-ray properties PSR B0943+10. When pulsar a sustained 'bright' mode, X-rays show only an un-pulsed, non-thermal component. Conversely,...
The aim of the LOFAR Epoch Reionization (EoR) project is to detect spectral fluctuations redshifted HI 21cm signal. This signal weaker by several orders magnitude than astrophysical foreground signals and hence, in order achieve this, very long integrations, accurate calibration for stations ionosphere reliable removal are essential. One prospective observing windows EoR will be centered at North Celestial Pole (NCP). We present results from observations NCP window using highband antenna...
The location of radio pulsars in the period-period derivative (P-Pdot) plane has been a key diagnostic tool since early days pulsar astronomy. Of particular importance is how evolve through P-Pdot diagram with time. Here we show that decay inclination angle (alpha-dot) between magnetic and rotation axes plays critical role. In particular, alpha-dot strongly impacts on braking torque, an effect which largely ignored previous work. We carry out simulations include negative term, it possible to...
Context. The LOw Frequency ARray (LOFAR) radio telescope is a giant digital phased array interferometer with multiple antennas distributed in Europe. It provides discrete sets of Fourier components the sky brightness. Recovering original brightness distribution aperture synthesis forms an inverse problem that can be solved by various deconvolution and minimization methods Aims. Recent papers have established clear link between nature interferometry measurement "compressed sensing" (CS)...
We present the discovery of PSR J0250+5854, a radio pulsar with spin period 23.5 s. This is slowest-spinning known. J0250+5854 was discovered by LOFAR Tied-Array All-Sky Survey (LOTAAS), an all-northern-sky survey for pulsars and fast transients at central observing frequency 135 MHz. subsequently detected pulsations from in interferometric images Two-meter Sky Survey, allowing subarcsecond localization. This, along pre-discovery detection 2 years prior, allowed us to measure spin-period...
We report the detection of 48 millisecond pulsars (MSPs) out 75 observed thus far using LOFAR in frequency range 110-188 MHz. have also detected three MSPs nine 38-77 This is largest sample ever at these low frequencies, and half were for first time frequencies below 200 present average pulse profiles MSPs, their effective widths, flux densities compare with higher observing frequencies. The flux-calibrated, multifrequency are publicly available via EPN Database Pulsar Profiles. values...
Fast radio bursts (FRBs) are millisecond signals that exhibit dispersion larger than what the Galactic electron density can account for. We have conducted a 1446 h survey for FRBs at 145 MHz, covering total of 4193 deg2 on sky. used UK station low frequency array (LOFAR) telescope – Rawlings Array accompanied majority time by LOFAR Nançay, observing same fields frequency. Our real-time search backend, Advanced Radio Transient Event Monitor and Identification System artemis, utilizes graphics...
We present an overview of the LOFAR Tied-Array All-Sky Survey (LOTAAS) for radio pulsars and fast transients. The survey uses high-band antennas Superterp, dense inner part core, to northern sky (dec > 0 deg) at a central observing frequency 135 MHz. A total 219 tied-array beams (coherent summation station signals, covering 12 square degrees), as well three incoherent (covering 67 degrees) are formed in each pointing. For ofthe 222 beams, intensity is recorded 491.52 us time resolution. Each...
We determined Faraday rotation measures (RMs) towards 137 pulsars in the northern sky, using Low-Frequency Array (LOFAR) observations at 110-190 MHz. This low-frequency RM catalogue, largest to date, improves precision of existing measurements on average by a factor 20 - due low frequency and wide bandwidth data, aided synthesis method. report RMs 25 for first time. The were corrected ionospheric increase accuracy our catalogue approximately 0.1 rad m$^{\rm -2}$. correction is currently...
We have used the central 44 antennas of new 64-dish MeerKAT radio telescope array to conduct a deep search for pulsars in core nine globular clusters. This has led discovery eight millisecond six different Two binaries, 47 Tuc ac and ad, are eclipsing "spiders", featuring compact orbits ($\lesssim 0.32$ days), very low-mass companions regular occultations their pulsed emission. The other three binary (NGC 6624G, M62G, Ter 5 an) wider ($> 0.7$ days) orbits, with that likely be white dwarfs or...
ABSTRACT Binary supermassive black hole (SMBH) systems result from galaxy mergers, and will eventually coalesce due to gravitational wave (GW) emission if the binary separation can be reduced ≲0.1 pc by other mechanisms. Here, we explore a self-lensing SMBH model for sharp (duration ∼1 h), quasi-regular X-ray flares – dubbed quasi-periodic eruptions recently observed two low-mass active galactic nuclei: GSN 069 RX J1301.9+2747. In our model, is ∼edge-on, such that each gravitationally lenses...
We present the first 2.5 years of data from MeerKAT Pulsar Timing Array (MPTA), part MeerTime, a Large Survey Project. The MPTA aims to precisely measure pulse arrival times an ensemble 88 pulsars visible Southern Hemisphere, with goal contributing search, detection and study nanohertz-frequency gravitational waves as International Array. This project makes use telescope, operates typical observing cadence two weeks using L-band receiver that records 856-1712 MHz. provide comprehensive...
The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is a radio interferometer array currently in development, with an initial 256-element to be deployed at the South African Radio Astronomy Observatory (SARAO) Square Kilometer Array (SKA) site Africa. Each of 6m, $f/0.23$ dishes will instrumented dual-polarisation feeds operating over frequency range 400-800 MHz. Through intensity mapping 21 cm emission line neutral hydrogen, HIRAX provide cosmological survey distribution...
MeerTime is a five-year Large Survey Project to time pulsars with MeerKAT, the 64-dish South African precursor Square Kilometre Array. The science goals for programme include timing millisecond (MSPs) high precision (< 1 $\mu$s) study Galactic MSP population and contribute global efforts detect nanohertz gravitational waves International Pulsar Timing Array (IPTA). In order plan remainder of use allocated most efficiently, we have conducted an initial census MeerKAT "L-band" receiver 189...
We present the largest single survey to date of average profiles radio pulsars, observed and processed using same telescope data reduction software. Specifically, we measurements for 1170 by Thousand Pulsar Array (TPA) programme at 64-dish SARAO MeerKAT telescope, in a frequency band from 856 1712 MHz. provide rotation measures (RM), dispersion measures, flux densities polarization properties. The catalogue includes 254 new RMs that substantially increase total number known pulsar RMs. Our...
In spite of the rich phenomenology polarization properties radio pulsars, rotating vector model (RVM) created 50 years ago remains best method to determine beam geometry a pulsar. We apply RVM sample 854 pulsars observed with MeerKAT telescope in order draw conclusions about population as whole. The main results are that (i) geometrical interpretation position angle traverse is valid majority population, (ii) for which fails tend have high fraction circular compared linear polarization,...
We measured a sample of 150 pulsar rotation measures (RMs) using the 20-cm receiver Parkes 64-m radio telescope. 46 pulsars in our have not had their RM values previously published, whereas 104 RMs been revised. used novel quadratic fitting algorithm to obtain an accurate from calibrated polarization profiles recorded across 256 MHz bandwidth. The new data are conjunction with known dispersion and NE2001 electron-density model study models direction magnitude Galactic magnetic field.
M87 is a giant elliptical galaxy located in the centre of Virgo cluster, which harbours supermassive black hole mass 6.4x10^9 M_sun, whose activity responsible for extended (80 kpc) radio lobes that surround galaxy. The energy generated by matter falling onto central ejected and transferred to intra-cluster medium via relativistic jet morphologically complex systems buoyant bubbles, rise towards edges halo. Here we present first observations made with new Low-Frequency Array (LOFAR) at...